Gasoline may be produced from refining crude oil through several processes comprising distillation and cracking. The synthesis of gasoline may also be effected by catalytic conversion of easily convertable oxygenates such as methanol or methanol and dimethyl ether (methanol/dimethyl ether) over zeolites, e.g. ZSM-5, at temperatures between 300° C. and 600° C. and pressures from atmospheric to few hundred bars, preferably from atmospheric to 100 bar. Typical WHSV (weight hour space velocity) of C1 oxygenate equivalents is in the range 0.2-10. The catalytic conversion of methanol to gasoline is described in more detail by Chang in “Methanol to Hydrocarbons”, Catal. Rev. 25 (1983)1.
Although the conversion of methanol or methanol/dimethyl ether into gasoline is generally referred to as the Methanol-to-Gasoline (MTG) process, oxygen-containing hydrocarbons (oxygenates) other than methanol are easily converted in the MTG process. The group of easily convertible oxygenates is not confined to methanol and/or dimethyl ether (DME), but comprises apart from alcohols and ethers, esters, long chain aldehydes, ketones and their analogues as described in U.S. Pat. No. 3,998,898. Apart from the desired portion of especially C5+ gasoline products and co-produced water gasoline synthesis results in some by-production of olefins, paraffins, methane and products from thermal cracking (hydrogen, CO, CO2). Subsequent separation and/or distillation ensures the upgrading of the raw hydrocarbon product mixture to useful gasoline. Naturally, a high yield of the useful gasoline products is desirable for obtaining proper process economy.
In the synthesis of gasoline according to the MTG process the basic feed is methanol or other oxygenates which are evaporated and then introduced to the process. In a variant of the MTG process the basic feed is synthesis gas which in a first step is partly converted into methanol or methanol/dimethyl ether and, in a second step, the methanol or methanon/dimethyl ether is converted into gasoline and unconverted synthesis gas is recycled to the first step. In other words, the synhesis of methanol or methanol/dimethyl ether is integrated with the synthesis of gasoline. The integrated process is described in more detail by Topp-Jørgensen in Stud. Surf. Sci. Catal. 36 (1988) 293.
In this integrated process methanol is synthesized from synthesis gas, i.e. gas containing mainly H2, CO and CO2 according to the following reaction schemes:CO2+3H2CH3OH+H2OCO+H2OCO2+H2 while the synthesis of DME is conducted by the dehydration of methanol according to the following scheme:2CH3OHDME+H2O
Depending on the operating conditions more or less by-product is formed, primarily small amounts of higher alcohols, ketones, aldehydes and acids. Catalysts for the synthesis of methanol and dimethyl ether are well known in industrial practice. Typical examples of methanol synthesis catalysts are mixtures of Cu/Zn/Al oxides and, for dimethyl ether synthesis, acid catalysts, e.g. silica-alumina. The catalysts may be applied separately to convert synthesis gas to methanol and to convert methanol to dimethyl ether, respectively, or they may be combined to produce mixtures of methanol and dimethyl ether directly from synthesis gas. The combination may be effected by physically mixing the catalysts or they may be co-fabricated to combine all of the above catalytic functions in one and the same catalyst. It is characteristic to the class of Cu/Zn/Al-based methanol synthesis catalysts that they are also efficient hydrogenation catalysts.
The combined synthesis of methanol/dimethyl ether in the first step of the integrated process is normally preferred rather than synthesis of methanol only as the further conversion of methanol to DME in said first step reduces the heat evolved, and thereby the operating temperature in the second subsequent step, i.e. the gasoline synthesis section, which in turn ensures a higher yield of gasoline product and/or a cheaper gasoline synthesis.
It is known that, in the integrated gasoline synthesis process, synthesis gas containing hydrogen, carbon monoxide, carbon dioxide, proper in composition for the synthesis of methanol and/or DME, and inert components is mixed with a recycle stream from a separation step subsequent to the gasoline synthesis section. The recycle stream contains unconverted synthesis gas and volatile products from the gasoline synthesis. The admixture of synthesis gas and recycle stream is heated and passed to the methanol or methanol/DME synthesis section. The effluent from the methanol or methanol/DME synthesis section is normally mixed with a second recycle stream from the separator subsequent to the gasoline synthesis section to obtain a mixed feed containing easily convertible oxygenates, which is then passed to the gasoline synthesis section. The gasoline synthesis takes place in well known fixed bed and/or fluidised bed reactors. The effluent from the gasoline synthesis section and which is enriched in gasoline components and water, light olefinic hydrocarbons, methane and paraffins is cooled and passed to a separating unit where water, hydrocarbons and unconverted synthesis gas containing i.a. volatile hydrocarbons and hydrogen are separated, the latter stream normally being split in a purge stream, a first recycle stream and a second recycle stream. Thus, in the integrated gasoline synthesis process hydrogen, carbon monoxide and carbon dioxide are present in significant amounts in the methanol containing feed stream which enters the gasoline synthesis section. This process is normally referred to as TIGAS (Topsøe Integrated Gasoline Synthesis).
The conversion of easily convertible oxygenates such as methanol to gasoline as a stand-alone process is for instance known from U.S. Pat. No. 3,998,898. This citation teaches that difficultly convertible aliphatic oxygenates comprising the group of short chain aldehydes, carboxylic acids and anhydrides, glycols, glycerine and carbohydrates may be converted to gasoline products, though in a less satisfactory manner and with poorer catalyst cycle life as compared to the easily convertible oxygenates. Yet by co-feeding to the gasoline reactor easily convertible oxygenates with difficultly converted oxygenates at a temperature of at least 260° C. and a space velocity of 0.5 to 50 LHSV (Liquid Hour Space Velocity) a higher yield of gasoline is obtained than when either reactant type is used alone. However, by co-feeding to the gasoline reactor the difficultly convertible oxygenates a substantial fraction of the oxygenate carbon ends up as carbon oxides in the effluent from the gasoline reactor, i.e. the ratio of carbon in the hydrocarbon product with respect to the carbon in the feed to the gasoline reactor decreases. Part of the carbon loss as carbon oxides is related to the cracking of hydrocarbons, thus creating undesired attendant effects such as reduced catalyst cycle time in the gasoline reactor.
It is well known that the catalytic conversion of oxygenates into gasoline over acid catalysts like zeolites is accompanied by the formation of carbonaceous deposits, generally referred to as “coke” which is undesirable because it deactivates the catalyst, and because it represents a loss of carbon value. The coke formation rate depends inter alia on the zeolite applied, the feed components and on the operating conditions in particular temperature. The catalytic coke must be removed from the catalyst to regain catalytic activity, typically by burning off the coke under controlled conditions.
Apart from the coke formation associated with the catalytic conversion of oxygenates into gasoline also thermal cracking of less stable oxygenates (difficultly convertible) may lead to coke formation not only depositing on the catalyst surface but also on reactor internals, in heat exchangers, valves and other equipment.
Coke associated with the catalytic conversion, formed only on the interior and exterior surface of the catalyst, must be distinguished from the coke formed by thermal cracking of less stable oxygenates, e.g. by condensation/polymerisation of precursors which may take place when preheating and introducing the oxygenate feed to the conversion catalyst and which results in coke lay-down on the external surface of the catalyst particles (pellets, extrudates, etc.) and elsewhere on the surface of reactor internals, exchangers, valves, etc. The formation of coke due to thermal cracking is thus particularly undesirable because it causes plugging not only in the reactor but also in the feed system upstream the catalyst bed, i.e. in equipment which does not tolerate the excessive temperatures associated with removal of coke by controlled burn-off.
The catalyst cycle time as defined herein is the length of the period, wherein the catalyst exhibits proper catalytic activity before the catalyst must be regenerated by burning off the coke. Short catalyst cycle time means that an expensive type of reactor must be employed e.g. with continuous regeneration of catalyst circulated between reactor and regenerator or that several reactors in parallel must be employed with frequent shifts in operation mode (synthesis or regeneration) and being equipped with complex control. An increased catalyst cycle time benefits the process by a reduction in investment and improved process efficiency.
Apart from a process with increased catalyst cycle time it would also be desirable to be able to provide a process which is less sensitive (i.e. more robust) to changes in co-feed composition, so that the co-feed to the integrated gasoline process is able to treat a wide range of difficultly convertible oxygenates, including particularly difficultly convertible aromatic compounds and short-chained aldehydes, such as phenol, 2-methoxy phenol (anisol) and acetaldehyde.
Examples of substances containing difficultly convertible oxygenates of interest for the co-feeding into a gasoline synthesis are e.g. bio-oil products prepared by high pressure liquefaction or by pyrolysis.
Bio-fuels have found interest due to their CO2 neutrality to the environment, thus the CO2 released during combustion corresponds to the amount consumed through the growth of the plant of origin, e.g. wood. Sources of bio-oils are forest and agricultural waste such as sawdust, bark, or sugar cane waste bagasse. Bio-oil products have been shown to contain highly oxygenated compounds (difficultly convertible compounds), the exact composition of which depends upon the type of raw material source and processing conditions. The bio-oil product is furthermore rather instable with respect to secondary reactions such as condensation and polymerisation making bio-oils of limited potential as fuels. In particular, the presence of difficultly convertible aromatic compounds and short-chained aldehydes, such as phenol, 2-methoxy phenol (anisol) and acetaldehyde, has prevented the otherwise appealing utilisation of bio-oil in gasoline synthesis.
For instance, in “Transformation of Oxygenate Components of Biomass Pyrolysis Oil on a HZSM-5 zeolite” I&II (Ind. Eng. Chem. Res., Vol. 43, No. 11, 2004) by Gayubo et al. the conversions of selected model components over HZSM-5 was investigated. It was found, in conclusion, that short chain aldehydes and aromatics, such as acetaldehyde and phenol and 2-methoxy phenol (anisol), all of which are common constituents in bio-oil, exhibited low reactivity towards gasoline production and caused severe coke formation. Hence, due to the problems associated with said oxygenates it is normally considered more advantageous that phenols and aldehydes, as materials of economic interest, be separated from the oxygenate mixture before being passed over a zeolite for gasoline production.
In “Catalytic co-processing of biomass-derived pyrolysis vapours and methanol” by Horne et al. (J. of Analytical and Applied Pyrolysis, 34 (1995) 87-108)) various ratios of methanol and bio-oil were used in the co-feed of a hydrocarbon process conducted over a zeolite (ZSM-5) catalyst. The amount of coke generated by the bio-oil, containing a complex mixture of oxygenates, showed to be more than 4 times the amount of coke generated by feeding 100% methanol at 500° C. The coke formation was found to be a linear function of the fraction of bio-oil (balanced by methanol).
Bio-oil is made up of two classes of compounds originating from the cellulose and lignin parts of the pyrolysed biomass. The lignin break-down components are phenol derivatives, while the cellulose degradation part consists of mainly aldehydes, hydroxyaldehydes and alcohols and acids, typical components being furfural, furfuryl alcohol and short chain aldehydes such as acetaldehyde, hydroxyacetaldehyde, glyoxal, glycolic acid and acetol. The unsaturated nature of the product mixture may explain its relatively poor thermal stability, i.e. the bio-oil polymerizes and causes clogging upon heating. If the bio-oil is to be converted to a desirable gasoline fuel product in a manner where the fuel value of the bio-oil is largely maintained and where its instability is overcome the usefulness and thereby the value of bio-oil will be increased.
It is known that in the conversion of methanol and methanol/dimethyl ether over ZSM-5 and similar zeolites, durene (1,2,4,5-tetramethylbenzene) forms in relatively high amounts in the gasoline synthesis from methanol/dimethylether over HZSM-5. If present in the gasoline in excessive amounts (above 4-7 percent weight) durene may cause clogging of the fuel system in cold weather, in particular it may cause problems in the engine carburator system. Thus, excessive durene formation is undesirable in the gasoline synthesis. Durene may be removed from the gasoline product by subjecting a higher-boiling fraction, rich in durene, to a mild hydrocracking. This, however, incurs an additional cost of processing. It is generally believed that the formation of durene is favoured by increased pressure and therefore operation at the lowest possible pressure is normally required. It would be desirable that the level of durene in the gasoline product be kept at a low level as an increased removal of durene gives rise to both increased investment and loss of valuable gasoline product.